The use of syringe pumps as driving means for pipetting slight quantities of liquids is common. As advanced syringe pumps are based on stepping motors with a high resolution the high-precision manipulation of liquid quantities in the nano litre range is possible. Common needle-type pipetting devices comprise a single channel consisting of a syringe pump fixedly mounted in the housing, which is connected via a hose filled with the liquid to the three-dimensionally displaceable pipetting needle. As the connecting hose may be deformed when the needle is displaced and as its volume does hence not remain constant the minimum pipetting volume is limited to the micro litre range. Such devices are used, for instance, for hit picking in HTS applications where samples from isolated wells of a major number of micro titre plates are combined on a new micro titre plate. As each micro titre plate must be serially processed needle-type pipetting devices are often equipped with several pipetting channels. When each channel must be separately controlled a corresponding high number of syringe pumps are required. Large-size systems of this kind are provided with 96 channels, for instance, whereof respective groups of 8 are operated in parallel by means of one syringe pump. The installation of 96 syringe pumps is not possible for reasons of space and costs.
Equipment in which an array of pipettes is operated on the plunger stroke principle constitutes one alternative of the needle-type pipetting devices. In configurations with a single plunger, the samples are manipulated via an air cushion. In view of the size of the air cushion between the plunger and the samples, the minimum volume that can be pipetted with a high precision is limited to the micro litre range. Other configurations consist of an array composed of individual syringes.
As only small air cushions are created in each syringe the transfer of distinctly smaller quantities is possible. For example, a minimum pipetting volume of 100 nano litres is quoted for these devices. All the devices of this type share the common aspect that the pipettes cannot be controlled individually but operate always in parallel. Their principle field of application is therefore the HTS domain where as many samples as possible must be transferred and measured at a constant volume with a maximum rate from one micro titre plate to another plate. The pipetting precision remains comparatively low, which restricts the application to tests of a predominantly qualitative nature.